J. Daniels

Multistage coupling of independent laser-plasma accelerators

Laser-plasma accelerators (LPAs) are capable of accelerating charged particles to very high energies in very compact structures. In theory, therefore, they offer advantages over conventional, large-scale particle accelerators. However, the energy gain in a single-stage LPA can be limited by laser diffraction, dephasing, electron-beam loading and laser-energy depletion. The problem of laser diffraction can be addressed by using laser-pulse guiding and preformed plasma waveguides to maintain the required laser intensity over distances of many Rayleigh lengths; dephasing can be mitigated by longitudinal tailoring of the plasma density; and beam loading can be controlled by proper shaping of the electron beam. To increase the beam energy further, it is necessary to tackle the problem of the depletion of laser energy, by sequencing the accelerator into stages, each powered by a separate laser pulse. Here, we present results from an experiment that demonstrates such staging. Two LPA stages were coupled over a short distance (as is needed to preserve the average acceleration gradient) by a plasma mirror. Stable electron beams from a first LPA were focused to a twenty-micrometre radius—by a discharge capillary-based active plasma lens—into a second LPA, such that the beams interacted with the wakefield excited by a separate laser. Staged acceleration by the wakefield of the second stage is detected via an energy gain of 100 megaelectronvolts for a subset of the electron beam. Changing the arrival time of the electron beam with respect to the second-stage laser pulse allowed us to reconstruct the temporal wakefield structure and to determine the plasma density. Our results indicate that the fundamental limitation to energy gain presented by laser depletion can be overcome by using staged acceleration, suggesting a way of reaching the electron energies required for collider applications.

Active Plasma Lensing for Relativistic Laser-Plasma-Accelerated Electron Beams

Compact, tunable, radially symmetric focusing of electrons is critical to laser-plasma accelerator (LPA) applications. Experiments are presented demonstrating the use of a discharge-capillary active plasma lens to focus 100-MeV-level LPA beams. The lens can provide tunable field gradients in excess of 3000 T/m, enabling cm-scale focal lengths for GeV-level beam energies and allowing LPA-based electron beams and light sources to maintain their compact footprint. For a range of lens strengths, excellent agreement with simulation was obtained.

Generation and pointing stabilization of multi-GeV electron beams from a laser plasma accelerator driven in a pre-formed plasma waveguidea)

Laser pulses with peak power 0.3 PW were used to generate electron beams with energy >4 GeV within a 9 cm-long capillary discharge waveguide operated with a plasma density of ≈7×1017 cm−3. Simulations showed that the super-Gaussian near-field laser profile that is typical of high-power femtosecond laser systems reduces the efficacy of guiding in parabolic plasma channels compared with the Gaussian laser pulses that are typically simulated. In the experiments, this was mitigated by increasing the plasma density and hence the contribution of self-guiding. This allowed for the generation of multi-GeV electron beams, but these had angular fluctuation ≳2 mrad rms. Mitigation of capillary damage and more accurate alignment allowed for stable beams to be produced with energy 2.7±0.1 GeV. The pointing fluctuation was 0.6 mrad rms, which was less than the beam divergence of ≲1 mrad full-width-half-maximum.

Plasma density diagnostic for capillary-discharge based plasma channels

The plasma density in discharged laser guiding structures, of order 1018 cm−3, is critical to laser-plasma accelerators. Here, we demonstrate a technique that uses spectral interferometry to measure the on-axis laser group velocity (and thus density) in cm-scale cylindrical hydrogen-discharge plasma channels by using laser pulses with a Gaussian transverse profile. Experimental density retrieval over a range of capillary parameters (density, length, and diameter) is presented. The accuracy (of order 8×1016 cm−3) and shot-to-shot stability (of order 2×1016 cm−3) of the diagnostic are discussed.

Staging of independent laser plasma accelerators

We present results of an experiment where two independent Laser-Plasma-Accelerator (LPA) stages are coupled at a short distance by a plasma mirror. Changing the arrival time of the electron beam with respect to the second-stage laser pulse allowed reconstruction of the temporal field structure and determination of the plasma density. Injection into the wakefield of the second stage was verified by a 100 MeV energy gain of the electron beam.

Plasma control and diagnostics for 10 GeV electron beams on BELLA

To advance the current state-of-the-art of capillary-based laser plasma accelerators (LPAs), the tunability of capillary discharge plasma channels needs to be improved. We present the techniques used to determine critical properties of the plasma density distribution. Independent tailoring of plasma channel width and on-axis density are required to produce higher energy electron beams with existing facilities. A scheme involving an additional, nanosecond laser pulse to locally heat the channel has been proposed previously. We discuss recent progress on the implementation of this scheme, demonstrating a heating effect on the plasma channel as evidenced from nanosecond-resolved spectroscopy on transversely emitted plasma light. PIC simulations indicate the possibility of accelerating high charge beams up to 8.4 GeV average energy if other technique advances are made as well. These include the need for longer plasma channels of 10s of centimeters, low plasma density and an ionization injection scheme to inje...

Characterization of the spectral phase of an intense laser at focus via ionization blueshift

An in situ diagnostic for verifying the spectral phase of an intense laser pulse at focus is presented. This diagnostic relies on measuring the effect of optical compression on ionization-induced blueshifting of the laser spectrum. Experimental results from the Berkeley Lab Laser Accelerator, a laser source rigorously characterized by conventional techniques, are presented and compared with simulations to illustrate the utility of this technique. These simulations show distinguishable effects from second-, third-, and fourth-order spectral phase.

Plasma channel diagnostics for capillary discharges

The plasma properties of a plasma waveguide are critical to the performance of laser-plasma accelerators (LPAs). By measuring the group velocity in plasma channels through spectral interferometry, the density in the channel is retrieved. In this paper, experimental methods and results are presented for the plasma density in LPA-relevant plasma channels of various lengths.

Laser-assisted capillary discharge for enhanced guiding of tightly focused laser pulses at low densities

Laser-plasma accelerators (LPAs) rely on intense laser fields that create wakes in plasmas. Advancement in the field of LPAs depends on extending the laser-plasma interaction length. State-of-the-art accelerators make use of laser guiding by capillary discharge channels. The transverse density profile (channel depth) of such channels confines the laser, and the on-axis density determines the energy transfer to the wake. The transverse profile can be controlled by choosing the radius of the capillary, but laser-induced capillary damage occurs when the radius is reduced to achieve the required channel depth. Both the on-axis density and the transverse profile depend on the pressure inside the capillary before discharge. As the pressure is reduced to increase the interaction length, confinement of the laser beam is reduced. A scheme to improve laser guiding at low densities by locally heating the plasma with a secondary, nanosecond-scale heater laser has been implemented, and preliminary results are presented here. Heating of the plasma and modified confinement of the main laser pulse have been demonstrated.

Staged acceleration experiments

We present initial experiments on staging of two separately driven laser plasma accelerators (LPAs) towards high energy physics and beam deceleration experiments. A study establishing long term stability of electron beams accelerated by an LPA in density downramp configuration [1] is presented, demonstrating the appropriateness of these beams as an injector for staged acceleration. Subsequently, these injector beams are used to longitudinally probe the fully characterized wakefield [2] excited in a discharge-capillary-based second stage accelerator.